1. Field of the Invention
The present invention relates to a sizing composition for composite yarns and to composite yarns coated with this composition.
2. Discussion of the Background
Composite yarns (also called hybrid yarns) are yarns composed of organic filaments and glass filaments and are used for the production of composite products. These yarns are mainly obtained by combining together glass filaments and thermoplastic filaments directly under the die so as to obtain good mixing ("intimate" mixing) of glass filaments and thermoplastic filaments. The processes for obtaining composite yarns directly at the die are specially described in Patents EP-A-O 367 661 (U.S. Pat. No. 5,011,523), EP-A-O 505 275 (U.S. Pat. No. 5,328,493), EP-A-O 505 274 (U.S. Pat. No. 5,316,561), WO 93/15893, EP-A-O 599 695 and EP-A-O 616 055. Before combining together the glass filaments and thermoplastic filaments under the die, the glass filaments are coated with a sizing composition intended, especially, to protect the yarns from abrasion. This sizing composition is usually an aqueous composition (it generally contains more than 90% by weight of water), the aqueous compositions being easier to handle and conventionally suitable for coating glass filaments (this type of composition especially having a very high wetting power with respect to glass filaments).
However, thermoplastics have little or no affinity with water and their hydrophobic nature leads to a phenomenon of repulsion between the sized glass filaments and the thermoplastic filaments. This phenomenon is accentuated during the drying which is generally carried out on composite yarns in the case of use of an aqueous sizing composition (in particular, drying enables the composite yarns to be used more easily in conversion processes, such as extrusion processes). The glass filaments then tend to group together as the water progressively evaporates, the thermoplastic filaments being ejected to the outside of the composite yarn. Instead of obtaining the desired intimate mixing of glass filaments and thermoplastic filaments within the composite yarns, a segregation of the thermoplastic filaments is therefore observed, with partial or complete lack of cohesion of the hybrid yarn, the thermoplastic filaments in particular ending up without any protection from the risks of abrasion or electrostatic phenomena.
The segregation of the thermoplastic filaments and their absence of protection from abrasion pose problems, in particular when using the composite yarns in textile applications, for example in weaving in which the yarns rub on feed rollers. In such cases, the composite yarns coated with aqueous size tend to break and to disrupt the running of the textile machines. The poor mixing between the glass filaments and thermoplastic filaments, associated with the segregation resulting from the use of a conventional aqueous sizing composition, is moreover observed in composites obtained from composite yarns comprising the said glass filaments and the said thermoplastic filaments, these composites not having a very satisfactory surface appearance.